The instant invention relates to improved electrically conductive substrates specifically designed for use in electrophotographic imaging processes. The improved substrate of the instant invention is fabricated from a non-deformable, electrically conductive metallic material, such as stainless steel, the deposition surface of which is "electrolevelled" so as to be characterized by a decreased number of surface defects, which defects can deleteriously effect the glow discharge deposition of the layers of semiconductor alloy material from which the electrophotographic photoreceptor is fabricated. By so forming the substrate, the morphological growth of the layers of semiconductor alloy material thereupon is improved due to the level, defect-free topology of the deposition surface thereof and the substrate becomes less susceptible to damage.
Electrophotography, also referred to generically as xerography, is an imaging process which relies upon the storage and discharge of an electrostatic charge by a photoconductive material for its operation. A photoconductive material is one which becomes electrically conductive in response to the absorption of illumination; i.e., light incident thereupon generates electron-hole pairs (referred to generally as "charge carriers"), within the bulk of the photoconductive material. It is these charge carriers which permit the passage of an electrical current through that material for discharge of the static electrical charge (which charge is stored upon the outer surface of the electrophotographic media in the typical electrophotographic process).
First the structure and then the operation of a typical xerographic or electrophotographic photoreceptor will be explained so that the operation and advantages of the instant invention may be fully appreciated. It is to be noted, however, that the improved enhancement layer of the instant invention is not limited to use with "typical" photoreceptors, but is equally adapted to be used with any photosensitive material which undergoes a change in any characteristic thereof under the influence of electromagnetic radiation, which characteristic provides for said material to have image reproduction capabilities.
As to the structure: A typical photoreceptor includes a cylindrically-shaped, electrically conductive substrate member, generally formed of a metal such as aluminum. Other substrate configurations, such as planar sheets, curved sheets or metallized flexible belts may likewise be employed. The photoreceptor also includes a photoconductive layer, which as previously described, is formed of a photoresistive material having a relatively low electrical conductivity in the dark and a relatively high electrical conductivity under illumination. Disposed between the photoconductive layer and the substrate member is a blocking layer, formed either by the oxide naturally occuring on the substrate member, or from a deposited layer of semiconductor alloy material. As will be discussed in greater detail hereinbelow, the blocking layer functions to prevent the flow of unwanted charge carriers from the substrate member into the photoconductive layer in which layer they could then neutralize the charge stored upon top surface of the photoreceptor. A typical photoreceptor also generally includes a top protective layer disposed upon the photoconductive layer to stabilize the electrostatic charge acceptance against changes due to adsorbed chemical species and to improve the photoreceptor durability. Finally, a photoreceptor also may include an enhancement layer operatively disposed between the photoconductive layer and the top protective layer, the enhancement layer adapted to substantially prevent charge carriers from being caught in deep traps and hence prevent charge fatigue in the photoreceptor.
In operation of the electrophotographic process: the photoreceptor must first be electrostatically charged in the dark. Charging is typically accomplished by a corona discharge or some other such conventional source of static electricity. An image of the object to be photographed, for example a typewritten page, is then projected onto the surface of the charged electrophotographic photoreceptor. Illuminated portions of the photoconductive layer, corresponding to the light areas of the projected image, become electrically conductive and pass the electrostatic charge residing thereupon through to the electrically conductive substrate thereunder, which substrate is generally maintained at ground potential. The unilluminated or weakly illuminated portions of the photoconductive layer remain electrically resistive and therefore continue to be proportionally resistive to the passage of electrical charge to the grounded substrate. Upon termination of the illumination, a latent electrostatic image remains upon the photoreceptor for a finite length of time (the dark decay time period). This latent image is formed by regions of high electrostatic charge (corresponding to dark portions of the projected image) and regions of reduced electrostatic charge (corresponding to light portions of the projected image).
In the next step of the electrophotographic process a fine powdered pigment bearing an appropriate electrostatic charge and generally referred to as a toner, is applied (as by cascading) onto the top surface of the photoreceptor where it adheres to portions thereof which carry the high electrostatic charge. In this manner a pattern is formed upon the top surface of the photoreceptor, said pattern corresponding to the projected image. In a subsequent step the toner is electrostatically attracted and thereby made to adhere to a charged receptor sheet which is typically a sheet of paper or polyester. An image formed of particles of toner material and corresponding to the projected image is thus formed upon the receptor sheet. In order to fix this image, heat and/or pressure is applied while the toner particles remain attracted to the receptor sheet. The foregoing describes a process which is the basis of many commercial systems, such as plain paper copiers and xeroradiographic systems.
It should be clear from the foregoing discussion that in order to obtain high resolution copies, it is desirable that the electrophotographic photoreceptor accept and retain a high static electrical charge in the dark; it must also provide for the flow of the charge carriers which form that charge from portions of the photoreceptor to the grounded substrate, or from the substrate to the charged portions of the photoreceptor under illumination; and it must retain substantially all of the initial charge for an appropriate period of time in the non-illuminated portions without substantial decay thereof. Image-wise discharge of the photoreceptor occurs through the photoconductive process previously described. However, unwanted discharge may occur via charge injection at the top or bottom surface and/or through bulk thermal charge carrier generation in the photoconductor material.
A major source of charge injection is at the metal substrate/semiconductor alloy material interface. The metal substrate provides a virtual sea of electrons available for injection and subsequent neutralization of, for example, the positive static charge on the surface of the photoreceptor. In the absence of any impediment, these electrons would immediately flow into the photoconductive layer; accordingly, all practical electrophotgraphic media include a bottom blocking layer disposed between the substrate and the photoconductive member. This bottom blocking layer is particularly important for electrophotographic devices which employ photoconductors with dark conductivities greater than 10.sup.-13 ohm.sup.-1 cm.sup.-1. As mentioned hereinabove, in some cases the blocking layer may be formed by native oxides occuring upon the surface of the substrate, as for example a layer of alumina occuring on aluminum. In other cases, the blocking layer is formed by chemically treating the surface of the substrate. An important class of blocking layers is formed by depositing a layer of semiconductor alloy material of appropriate conductivity type onto the substrate to give rise to blocking conditions.
In order to better understand the manner in which the blocking layers operate, it is necessary to review in greater depth a portion of the physics involved in the blocking layer phenomenon. As previously mentioned, the blocking layer must inhibit the transport and subsequent injection of the appropriate charge carrier (electrons for a positively charged drum) principally from the metal substrate into the body of the photoreceptor. This is accomplished in the doped semiconductor blocking layer by establishing a condition in which the minority charge carrier drift range, mu tau E, is smaller than the blocking layer thickness. Here, mu is the minority carrier mobility, tau is the minority carrier lifetime and E is the electric field strength. One can, for instance, substantially reduce the mu tau product for electrons by doping the blocking layer p-type. The excess holes present in the doped blocking layer greatly increase the probability of electron-hole recombination, thereby reducing the electron lifetime, tau. In effect a condition is achieved whereby electrons injected from the metal substrate recombine with holes in the p-type blocking layer before they are able to drift into the bulk of the photoreceptor to be swept through the top surface and neutralize the static charge thereon. However, while doping can serve to limit the mu tau product for the desired carrier, it can also give rise to deep electronic energy levels in the energy gap of semiconductor alloy material. This is particularly true for semiconductor alloy material, such as amorphous silicon alloys, in which the efficiency of substitutional doping is not high. These deep levels can become the source of thermally generated carriers or they can, if sufficiently numerous, provide a parallel path for the hopping conduction of electrons through the doped layers. Either of these phenomena can serve to compromise the blocking function of the doped layers.
In the course of operation of the typical electrophotographic process, described above, a positive corona charge is placed on the outer surface (the exposed surface of the top protective layer) of the electrophotographic media. The initial reaction of the photoconductive layer of the electrophotographic media to the application of this positive charge to the top surface thereof is to have any free electrons from the bulk be swept toward that surface in an attempt to neutralize the positive charge residing thereon. However, in the movement of these electrons from the bulk of the photoconductive layer to the outer surface of the top protective layer (on which surface the positive charge carriers have accumulated), said electrons encounter deep trap sites such as midgap defect states. While these trap sites are located throughout the bulk of the photoconductive layer, they are of particular importance when they reside near the interface of the photoconductive layer and the top protective layer. This is because the blocking function (the inability of the positive charge carriers electrostatically positioned on the periphery of the top protective layer to penetrate that layer) will cease to be effective (will "breakdown") when an electrical field of sufficient strength is placed across the top protective layer. Obviously, a given density of negative charge carriers trapped near the aforementioned interface of the top protective layer and the photoconductive layer will generate a sufficiently strong electrical field across the top protective layer to cause breakdown, whereas the same number of negative charge carriers trapped in the bulk thereof will not.
Further, trapping sites located deep in the energy gap of a semiconductor alloy material release trapped charge carriers at a much slower rate than do sites located closer to one of the bands. This results from the fact that more thermal energy is required, for example, to re-excite a trapped electron from the deep sites which exist near the middle of the energy gap to the conduction band than is required to re-excite an electron from the shallower sites which exist closer to the conduction band. The slow release rate from deep traps gives rise to a higher equilibrium trap occupancy and thus a higher electric field distribution.
It is important to note that in the fabrication of the typical electrophotographic photoreceptor which operates with a positive corona charge applied to outer surface thereof, the photoconductive layer thereof is made from a "pi-type" silicon:fluorine:hydrogen:boron alloy. As used herein, "pi-type" will refer to semiconductor alloy material, the Fermi level of which has been displaced from its undoped position closer to the conduction band to a position approximately "midgap". Further note that as used herein, the term "midgap" will be used to define a point in the energy gap of a semiconductor alloy material which is positioned approximately half-way between the valence band and the conduction band (in the case of 1.8 eV amorphous silicon:fluorine:hydrogen:boron alloy this is about 0.9 eV from each of the bands). It is necessary to make the photoconductive layer of the photoreceptor pi-type because the typical "intrinsic" amorphous silicon:hydrogen:fluorine alloy as deposited in a glow discharge decomposition process is slightly "nu-type" (the Fermi level of that material is slightly closer to the conduction band than to the valence band) and in a positive corona charge electrophotographic process, the movement of charge carriers through the photoconductive layer under illumination must be maximized while miminizing the thermal generation of charge carriers.
It is to be noted that when the Fermi level is positioned at midgap (as after the addition of the p-dopant to the silicon:fluorine:hydrogen alloy material), electrons moving through said pi-type material will encounter deep traps from which they cannot readily emerge. This is because the deepest electron trap sites in a layer of semiconductor alloy material lie at or near the Fermi level and in this Pi type material this energy coincides with midgap. The thermal energy required to release an electron from a deep trap is dependent on the depth of that trap. For a Fermi level position of 0.9 eV (midgap) the emission time has been calculated to be 4.times.10.sup.3 seconds at room temperature. This slow escape time means that it takes approximately 1.2 hours for an electron to vacate the trap. Obviously, an electrophotographic photoreceptor cannot tolerate such a slow electron discharge rate. If electrons, once trapped, remain confined for such a lengthy period of time, a large concentration of electrons trapped at the photoconductor layer/top protective layer interface will build up and this space charge and the positive charge accumulated on the surface of the top protective layer will create a very high electric field distortion across said top protective layer, which field causes the top protective layer to "breakdown". As used herein, "breakdown" refers to the inability of the top protective layer to inhibit the flow of charge carriers therethrough.
This breakdown phenomena can be eliminated by reducing the number of defect states which give rise to deep charge carrier traps. By positioning of the Fermi level of the semiconductor alloy material from which the enhancement layer is formed to a position above midgap, electrons moving through the enhancement layer do not have to pass through a region in which there are effective deep midgap traps. This translates into an electron escape time of less than about 1 second for a 1.8 eV silicon:hydrogen:fluorine:phosphine alloy having the Fermi thereof positioned in the most favored range of 0.75 to 0.65 eV from the conduction band. Because of the quick release time there will be no substantial build up of trapped charge in this region and therefore no high field distortion.
The semiconductor alloy material of the enhancement layer which is interposed between the photoconductive layer and the top protective layer is phosphorous doped in order to shift the Fermi level thereof toward the conduction band. By so shifting the Fermi level of the semiconductor alloy material, the electrons do not have to move through and become caught in the deep midgap states present in the energy gap thereof. This substantially eliminates the problems of charge fatigue by keeping the electrons out of the deep midgap states. Then both boron dopant and phosphorus dopant are introduced so as to pin the Fermi level at that preselected position in the energy gap through the addition of defect states on both sides of the pinned Fermi level. The added defect states, being shallow, not only solve charge fatigue problems, but those states are sufficiently numerous to inhibit lateral electron flow, quench the field effect and hence simultaneously solve image flow problems.
In light of the many definitions utilized for the terms "amorphous" and "microcrystalline" in the scientific and patent literature it will be helpful to clarify the definition of those terms as used herein. The term "amorphous", as used herein, is defined to include alloys or materials exhibiting long range disorder, although said alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions. As used herein the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occur. It is to be noted that pursuant to the foregoing definitions, the microcrystalline, materials employed in the practice of the instant invention fall within the generic term "amorphous" as defined hereinabove.
The concept of microcrystalline materials exhibiting a threshold volume fraction of crystalline inclusions at which substantial charges in key parameters occur, can be best understood with reference to the percolation model of disordered materials. Percolation theory, as applied to microcrystalline disordered materials, analogizes properties such as the electrical conductivity manifested by microcrystalline materials, to the percolation of a fluid through a non-homogeneous, semi-permeable medium such as a gravel bed.
Microcrystalline materials are formed of a random network which includes low mobility, highly disordered regions of material surrounding randomized, highly ordered crystalline inclusions or grains having high carrier mobility. Once these crystalline inclusions attain a critical volume fraction of the network, (which critical volume will depend, inter alia, upon the size and/or shape and/or orientation of the inclusions), it becomes a statistical probability that said inclusions are sufficiently interconnected so as to provide a low resistance current path through the network. Therefore at this critical or threshold volume fraction, the material exhibits a sudden increase in conductivity. This analysis (as described in general terms relative to electrical conductivity herein) is well known to those skilled in solid state theory and may be similarly applied to describe additional physical properties of microcrystalline materials, such as optical gap, absorption constant, etc.
The onset of this critical threshold value for the substantial change in physical properties of microcrystalline materials will depend upon the size, shape and orientation of the particular crystalline inclusions, but is relatively constant for different types of materials. It should be noted that while many materials may be broadly classified as "microcrystalline" those materials will not exhibit the properties Applicants have found advantageous for the practice of the subject invention unless they have a volume fraction of crystalline inclusions which exceeds the threshold value necessary for substantial change. Accordingly, we have defined "microcrystalline materials" to include only those materials which have reached the threshold value. Further note that the shape of the crystalline inclusions is critical to the volume fraction necessary to reach the threshold value. There exist 1-D, 2-D and 3-D models which predict the volume fraction of inclusions necessary to reach the threshold value, these models being dependent on the shape of the crystalline inclusions. For instance, in a 1-D model (which may be analogized to the flow of charge carriers through a thin wire), the volume fraction of inclusions in the amorphous network must be 100% to reach the threshold value. In the 2-D model (which may be viewed as substantially conically shaped inclusions extending through the thickness of the amorphous network), the volume fraction of inclusions in the amorphous network must be about 45% to reach the threshold value. And finally in the 3-D model (which may be viewed as substantially spherically shaped inclusions in a sea of amorphous material), the volume fraction of inclusions need only be about 16-19% to reach the threshold value: Therefore, amorphous materials, (even materials classified as microcrystalline by others in the field) may include crystalline inclusions without being microcrystalline as that term is defined herein.
Now that the structure and operation of a typical electrophotographic photoreceptor has been defined, it is possible to introduce the problem solved by the subject application, namely the problem of improving the morphological growth of semiconductor alloy material upon the deposition surface of a metallic substrate, such as an electrophotographic drum or web. In analyzing this problem, it is necessary to take into consideration the fact that the semiconductor alloy material must be deposited to a thickness which exceeds 25 microns and any surface imperfections on the substrate will drastically and deleteriously affect the growth of that material. The subject invention solves this problem of morphological growth by greatly reducing the number of surface defects so as to provide a morphologically level deposition surface upon which to grow said "thick" layers of semiconductor alloy material. The term "thick" has been placed in quotation marks because the total thickness of the layers of deposited semiconductor alloy material typically falls into the range of 15-30 microns (not much thicker than a human hair). Therefore, a surface defect can easily be propagated, and manifest its presence, through the entire thickness of the deposited layers of semiconductor alloy material. And even if the defect is not of sufficiently great size to be seen through those layers of material, it can still form a weak spot in the subsequently deposited semiconductor alloy material, said weak spot initiated by columnar growth which represents the preferred growth mechanism at defect sites. The columnar growth has a tendency to crack or peel when subjected to shear forces which occur when the electrophotographic photoreceptor is operatively employed and subjected to the abrasive force of copier paper continuously rolled thereagainst, the response of said weak spots to said continuous abrasion is to crack, which cracking results in the phenomenon known as "white spotting" in the copies made from such a photoreceptor.
Despite the use of the highest quality metals, such as aluminum or stainless steel to serve as the substrate or base electrode upon which the semiconductor material is deposited, it has been estimated that from 10,000 to 100,000 surface defects, i.e., irregularities, per square centimeter are present on the deposition surface thereof. Such irregularities take the form of projections, craters, or other deviations from a smooth finish, and may be under a micron in (1) depth below the surface, (2) height above the surface, or (3) diameter. Depending upon their configuration, size, the sharpness with which the irregularities deviate from a smooth surface finish, and the manner in which the semiconductor alloy material covers or fails to cover the defect, a weakened path through the semiconductor alloy material may be established, thereby effectively preventing the attainment of good blocking conditions, promoting white spotting and generally contributing to low saturation voltage capabilities in electrophotographic devices which has been fabricated thereupon. This may occur in numerious ways. For instance, a spike projecting from the surface of the substrate may be of too great a height to be covered by the subsequent deposition of the layers of semiconductor alloy material. Likewise, a crater formed in the surface of the substrate electrode may be of too large a diameter or too large a depth to be filled by the subsequent deposition of the layers of semiconductor alloy material. Note that even if the size of the defect (its deviation from a smooth surface) is not very large, but it includes one or more sharp or jagged features, said defect is still capable of causing the deposited semiconductor alloy material to be of less than optimum quality. This is because the sharp features of even small defects are capable of forming nucleation centers which promote nonhomogeneous and nonuniform growth of the deposited semiconductor alloy material, and (3) due to their presence, tend to initiate columnar growth which gives rise to the aforementioned weak spots.
Therefore, the instant invention is concerned with the elimination of defects which (1) due to the size thereof, cannot be adequately covered by the subsequent deposition of layers of that semiconductor alloy material, and (2) due to the sharp features thereof, inhibit the deposition of homogeneous, uniform layers of that semiconductor alloy material.
The instant invention, as will be described in greater detail hereinbelow, provides for the fabrication of electrophotographic photoreceptors which include an easily deposited substrate levelling layer for substantially eliminating surface defects inherently present in said substrates. More particularly, photoreceptor devices produced in accordance with the principles outlined by the subject disclosure are characterized by reduced white spotting and improved saturation voltage, which properties are achieved through the utilization of an electrolevelled substrate characterized by a substantial reduction of surface irregularities.
According to the principles of the preferred embodiment of the instant invention, a continuous, relatively thick, electrically conductive "levelling layer" is electroplated onto the deposition surface of the substrate so as to be operatively disposed between that substrate and the subsequently deposited body of semiconductor alloy material. This levelling layer functions to provide a smooth, substantially defect free deposition surface for that body of semiconductor alloy material. In this manner, the subsequently deposited body of semiconductor alloy material is able to uniformly, homogeneously and continuously cover the substrate, thereby substantially reducing problems associated with poor growth characteristics of the semiconductor alloy material. Accordingly, the instant invention provides an economical method for the manufacture of improved amorphous silicon, alloy based, thin film, large area electrophotographic photoreceptors characterized by substantially reduced white spotting, excellent current blocking capabilities, and hence by very high saturation voltages.
Yet another advantage can be derived from the utilization of the electrolevelled substrate of the subject invention. The material of choice from which metallic, electrically conductive substrates for electrophotographic photoreceptors are fabricated is aluminum. Aluminum is routinely employed because of the fact that it can be easily diamond polished so as to provide a high quality surface finish characterized by a relatively low number of surface defects. However, even the number of surface defects present on the surface of aluminum substrates is sufficiently high to cause the aforementioned problems with the morphological growth of the subsequently deposited layers of semiconductor alloy material. As discussed hereinabove, the levelling layer disclosed herein is capable of effectively removing a substantial percentage of the surface defects present on the surface of metallic substrate. Therefore, electrophotographic photoreceptors need no longer be fabricated from a material capable of being diamond polished.
Aluminum suffers from yet a further disadvantage, i.e., the fact that it is a relatively soft metal which is easily deformable. As a consequence of the commercial production and distribution of electrophotographic photoreceptors produced on aluminum drums, it has been found that said aluminum substrates are readily deformable and are unable to withstand the rough treatment inherent in the distribution of said photoreceptors. Prior to the instant invention, owing to the unavailability of a suitable alternative to the soft aluminum substrates, manufacturers of electrophotographic photoreceptors have been unable to fabricate said receptors from other, more durable metals.
However, through the application of the principles enunciated in the disclosure of the subject invention, it is now possible to fabricate electrophotographic photoreceptors from other, less expensive, more durable substrates which are subsequently electrolevelled. The result is the manufacture of a less expensive, very durable photoreceptor which also exhibits reduced stress in the deposited semiconductor alloy material and higher saturation voltages than were previously possible to attain in electrophotographic photoreceptors fabricated from aluminum substrates.
These and other advantages of the instant invention will become apparent from the drawings, the detailed description of the invention and the claims which follow.